1. Field of the Invention
The present invention relates to an adhesive coating for a medical instrument. More specifically, the invention relates to polymer compositions which, when applied to an insertable medical device, provide for improved adhesion of a coating to the surface of the device, and related methods.
2. Related Art
Medical devices such as catheters or guide wires are inserted through trachea, blood vessels, urethra or other celoms or tissues, or through catheters or drainage tubes etc. Such devices are required to have a high degree of smoothness to assure introduction of such devices without causing trauma to tissue encountered during placement. These surfaces may be further enhanced by having lubricity for preventing injury or inflammation of mucous membrane which would be caused when the devices remain in the tissue. Other requirements for medical device surfaces have also been recognized.
In some instances, it is advantageous for medical device surfaces to have the capability of serving as a depot for various physiologically active substances such as anti-thrombogenic substances, anti-microbial substances, anti-neoplastic substances, genetic materials, hormones, living cellular materials and others. Anti-thrombogenic materials, such as complexes of heparin with quaternary ammonium compounds, are used on medical device surfaces to prevent formation of blood clots on the surface, which can form rapidly on vascular prostheses in vitro. Antimicrobial agents including penicillins, cephalosporins, fluoroquinolones, aminoglycocides, silver, compounds, phenol compounds, biguanides and others have been proposed for use in surface coatings to control nosocomial infections that often occur on surfaces of implanted prostheses, U.S. Pat. No. 5, 069,899, U.S. Pat. No. 5,525,348, and U.S. Pat. No. 4,442,133.
The construction of devices such as guide wires and catheters presents special problems for insertion. Guide wires generally include coiled guide wires formed of stainless steel and monofilament guide which may have plastic materials such as polyurethanes, polyamides, plolyolefins, etc. extruded over them to provide a surface to which coatings can adhere, and to provide smoothness and uniformity of the surface.
Catheters typically consist of plastic tubes which may have a single lumen or multiple lumens. Catheters may have balloons fastened along the tube to obstruct a vessel or to fix the catheters in a desired position. Catheters may also have ports at the distal end, side ports along part of the length, or other mechanical features needed to accomplish the particular device mission. Catheters may consist of a continuous length of tubing, or may comprise two or more sections of tubing consisting of similar or dissimilar materials which are welded together in order to have different properties at different locations along the length of the device. Catheters may be tapered, both within a segment or by having segments of differing diameters. Typical material of which catheters are constructed include polyamides, polyurethanes, vinyls such as polyvinylchloride, polyesters, polyolefins, silicones, and others. Typical diameters range from less than one millimeter to more than 8 millimeters.
As typically encountered in inserting a catheter, at the predetermined site, the guide wire tip is inserted through a catheter up to its tip opening, the catheter with the guide wire is inserted into for example a blood vessel percutaneously, and the catheter is further inserted through the vessel by using the guide wire as a leading and supporting guide. These operations produce friction and abrasive forces that apply to the surfaces of the medical device. It is desirable for the frictional resistance between the catheter inner surface and the guide wire to be low. Relatively high friction between the catheter and the guide wire not only prevents the guide wire from being inserted through the catheter, but the guide wire from being easily moved through the catheter, making it difficult to carry out subtle indwelling operations at the destined vessel site. Sometimes the guide wire cannot be withdrawn from the catheter, rendering the catheter lumen unusable despite the completion of the indwelling operation.
To avoid such problems, attempts have been made in the prior art to apply low frictional resistance Teflon and silicone oil to the outer surface of guide wires. Application of silicone oil fails to retain lubricity because of immediate loss of silicone coatings. Frequent applications add to frictional resistance, also undesirably creating troubles as mentioned above.
There is thus the need for a catheter and guide wire having a lower frictional resistance surface which enables more subtle operation in a vessel and can be easily inserted and remain at the site where catheters are otherwise difficult to manage during placement.
Polyurethane coatings have been applied directly on metal surfaces. U.S. Pat. N0. 4,876,126. However, commercial versions of this technology require thick layers (60-80 microns thick) in order to perform adequately. In practice, the thick layer extends continuously around the coated metal substrate. These layers have good cohesive forces and thus appear to be tightly bound on the metal surface, even though these layers do not necessarily have good adhesion to the metal surface. A disadvantage of such coatings is that because the polyurethane and other plastic layers are so thick, the metal diameter of the underlying wire must be correspondingly diminished. This is especially troublesome on the very fine wires such as those used in coronary angioplasty or neurointerventional catheterization procedures. These wires have OD's of about 0.010" (about 250 microns) and may have the majority of the diameter (about 120 to 170 microns) composed of plastic materials instead of metals. An alternate method is the use of low frictional materials such as polytetrafluoroethylene coatings which have lower friction than metals and most other plastic materials and which can be applied directly onto metallic substrates. Other materials such as high density polyethylene have been tried, but the coefficients of friction are not low enough for such materials. Oils have been applied, and the coefficients of friction are low. However, such treatments are transient because they wear off during use.
Hydrogel coatings are known to provide a lubricious surface for insertable devices. However, metals and certain plastic materials such as polyolefins, polyamides, silicones, polyesters and some others have inert surfaces and it is often difficult to achieve acceptable adhesion when applying surface coatings, including hydrogel coatings, over such surfaces.
Hydrogels can absorb several times their weight in water when placed in an aqueous environment. Usually, hydrogel layers are attached to hydrophobic sublayer(s) and there may be a great deal of penetration of the hydrogel polymer molecules into the hydrophobic sublayer(s). The polymer molecules of both layers are left in a state of inter-molecular mingling, especially in the region of the interface between the two layers. As a result of the inter-molecular mingling, water that is taken up in the hydrogel may find its way to the intersection between the substrate and the hydrophobic coating layer. The adhesion between the hydrophobic layer and the substrate is usually jeopardized by the moisture, and adhesive failure usually results. This process of moisture-induced adhesive failure is greatly exacerbated when the coating layers are thin.
Thin hydrophobic layers containing cellulose esters and acrylic polymers may be coated directly on metal substrates, U.S. Pat. No. 5,001,009. Hydrogel coatings may be applied directly over such layers. Such systems perform well on coil type guide wires, because the coating is able to gain additional adhesion by penetrating between the coil wires. However, such layers tend to allow too much moisture penetration resulting in deterioration of adhesive bonds when applied onto mandril style metal substrates.